I strongly disagree that a living Jackson would have pulled off some miracle. Quite the contrary, the Jackson legend forgets that he made a critical, potentially war-ending mistake at Seven Pines, and that he could have done nothing more than Longstreet did (poor ol' Pete still gets blamed for Lee's foolishness).
To me the sign of a great commander is that he loses a smaller % of his forces than his enemy most of the time. In the Pacific in WW II, despite facing incredibly tough islands such as Iwo Jima, Okinawa, and others, Adm. Nimitz had the lowest ratio of casualties to total men employed in the entire war eclipsing the vaunted MacArthur. Well, Lee consistently had a higher % of casualties than his Union foes except at Fredericksburg---hard to lose that one!---and often lost the same total number of men as did Federal armies of about the same size. At Gettysburg, he lost a full 30% of all men committed to the action, an astonishing loss for a supposedly brilliant general. The supposedly inferior Union generals through the first 12 major battles or campaigns had a lower ratio of men lost to men committed than any southern general except at the aforementioned Fredericksburg, and leaving aside the obvious surrender at Vicksburg (a 100% loss to the Confeds that skews the data).
A union Corp was about the size of an oversized CSA Division. Union leadership was not up to snuff leading to smaller units/per officer.
The one thing Jackson could have done that Longstreet could not was to convince Lee that a flanking attack or redeployment would work.